The present invention relates to an improvement in semiconductor devices having a high density, especially large scale integrated circuits (LSI) of the type in which a printed circuit board over the upper surface of which are defined wiring patterns, is superposed over the upper surface of a metal base and the end portions of lead pins extended through the superposed printed-circuit board and the metal base and respectively connected to their corresponding wiring patterns.
In Dual Inline Packages (DIPs) which are a typical example of pin-insertion type semiconductor packages, the number of pins which can be provided is limited and in practice, is in the order of 60. It had been impossible so far to provide more pins than this. However, in order to provide a plurality of pins in excess of the order of 60, the pin-insertion type such as Pin Grid Array (a PGA) in which a plurality of pins are extended from the undersurface of the package has been used in general.
Various types of PGAs have been proposed and demonstrated. One is the so-called ceramic type in which a plurality of ceramic substrates are metallized and laminated into a multilayer construction. Another is the so-called plastic type PGA in which printed wiring substrates are replaced by a printed circuit board in order to achieve reductions in the fabrication costs.
However, the ceramic type PGA is generally very expensive and in the case of the plastic type PGA, there arises the problem that it is generally very difficult to completely seal the wiring circuit board and a semiconductor chip, a printed circuit board or the like air-tightly, so that when compared with the ceramic type PGA, the reliability of operation is inferior.
In view of the above, the same inventor proposed a semiconductor device of the type which has reliable operation and which is capable of reducing the manufacturing cost. This device is described in a pending disclosure in U.S. Ser. No. 095,256 (Japanese Patent Application Laid-Open No. 63-133553) in order to substantially solve the above and other problems encountered in the ceramic type PGA and the plastic type PGA.
That is, referring to FIGS. 1 and 2, a heat sink 2 is disposed at substantially the center portion of a metal base 1 and a semiconductor chip 3 is mounted on the upper surface of the heat sink 2. A printed circuit board 5 whose upper surface has predetermined wiring patterns 4 which are extended radially outwardly from the inner ends of the center portion around the semiconductor chip, is superposed over the upper surface of the base 1, and the inner ends of the center portion of the wiring patterns are electrically connected to the bonding pads of the semiconductor chip 3 by bonding wires 6, such as aluminum wires. When the base 1 and the board 5 are overlaid one upon the other, the upper end of the lead pin 7 is inserted into through holes 1a and 5a which are aligned with each other and insulating member 8 such as glass, is filled in the annular space defined between the through hole 1a of the base 1 and the lead pin 7 inserted thereinto. Over the upper surface of the printed circuit board 5, the wiring pattern 4 and the lead pin 7 are electrically interconnected with each other by a connecting member 9 such as solder. Furthermore, the upper surfaces of the semiconductor chip 3 and the printed circuit board 5 and their sides, are covered with a metal shell 10 so that the semiconductor chip 3 and the printed circuit board 5 are airtightly sealed.
In the case of the semiconductor device of the type described above, as shown in FIG. 5, the outer ends of the wiring patterns 4 defined over the upper surface of the printed circuit board 5 are extended to the outer peripheral end face. The reason why the device has been constructed above is described hereinafter. In the case that the wiring patterns 4 on the printed circuit board 5 are formed by copper (Cu) or the like which is a good conductor, bonding wires 6 such as aluminum wires are interconnected to the inner ends of the wiring patterns 4 formed by copper. In order to connect the bonding wire 6 such as aluminum wire to the inner end of the wiring patterns 4, the inner ends of the wiring patterns 4 are plated with gold so that the connection between the inner ends of the wiring patterns 4 and the bonding wires 6 can be securely stabilized.
That is, the production process of the conventional device will be described with reference to FIG. 4. Firstly, all the external ends of the wiring patterns 4 are extended into and around an external side of the printed circuit board 5 so that a patterning wiring layer 11 is formed in a size larger than the patterns 4 after their completion. Next, all the wiring patterns 4 are electrically short-circuited through the patterning wiring layer 11 so that the gold plating is accomplished. Lastly, after the gold plating is performed as shown in FIG. 5, the patterning wiring layer 11 is cut off so that production of the wiring patterns 4 is completed.
However, in the semiconductor device produced by the above-mentioned method for production, even though there is no problem in the case that the distance t1 between the outer peripheral side of the printed circuit board 5 and the inner surface of the upwardly extended portion of the metal shell 10 (FIG. 3) can be defined sufficiently large, but the following problems occur in making a semiconductor device as compact as possible. That is, when it is required to reduce the distance t1 as much as possible for the sake of compactness, electrical short-circuiting occurs between the external ends of the wiring patterns 4 by the contact of the inner surface of the upwardly extended portion of the metal shell 10 with one end of the wiring patterns 4. It follows therefore that there arises the problem that in order to prevent such electrical short-circuiting, the distance t1 must be inevitably increased so that the whole semiconductor device becomes larger in size.